An opening is provided in the cooling chimney connected to a mouth of a steel converter for a jetting lance, through which the lance can be lowered into the converter. This opening must be substantially larger than the diameter of the lance in order to remove possible caking formed on the lance caused by slag when the lance is raised through the opening so to have no mechanical contact therewith. Movable mechanical dosing means are conventionally arranged at the chimney opening for narrowing the gap between the chimney opening and the tubular outer surface area of the lance as far as possible, but without hindering the vertical movability of the lance, and for limiting the escape of hot dust-confining converter gases. Covers are also often arranged at the lance which lie on the chimney opening when lowered so as to close it. It is also conventional to provide a further impediment to the escape of gas in that blocking media such as stem or nitrogen are injected at high pressure at the chimney opening by means of nozzles in an annular arrangement.
The known lance barriers mentioned above are not gas tight, and therefore, let gas escape even as a result of slight internal overpressure or external air can penetrate into the system due to internal negative pressure in the system.